1. Field of the Invention
The present invention relates to an A/D converter, a battery pack, an electronics device and a method of voltage measurement and, more particularly, to technology which uses the A/D converter for measuring a voltage difference between both ends of a resistor element inserted in a current path in series, with the aim of current measurement.
2. Description of the Related Art
When a portable electronics device (a notebook personal computer, an electronic notebook or the like) is operated by a commercial power supply via an AC adapter or the like, the power supply is not interrupted during its operation. Meanwhile, when the portable electronics device is operated by a battery, there is the possibility of losing data under processing if a battery remaining capacity is used up and the power supply is interrupted during its operation. Therefore, a battery remaining capacity managing system for managing the battery remaining capacity is usually mounted on the portable electronics device.
In order to manage (predict) the battery remaining capacity, it is necessary to measure a current value of a current flowing out from the battery (a discharging current) or a current flowing into the battery (a charging current). A typical method of current measurement is to measure a voltage difference between both ends of a resistor element inserted in a current path in series, and determine the current value of the current path using the measured result and a resistor value of the resistor element. The battery remaining capacity managing system employs an A/D converter for measuring the voltage difference between both ends of the resistor element, and a microcontroller for determining the current value of the current path based on a digital value from the A/D converter (result of A/D conversion), in order to measure the current value of the current path.
Moreover, according to japanese Patent Application Laid-open No. Hei 6-90914, the technology is disclosed that a differential amplifier is provided at a pre-stage of an A/D converting circuit in order to eliminate DC noise included in an input signal to be subjected to A/D conversion, so that an overflow of the A/D converting circuit is prevented by applying the input signal to a non-inverting input terminal of the differential amplifier and applying a bias voltage to an inverting input terminal of the differential amplifier.
Current consumption of the portable electronics device varies significantly according to an operating state of the electronics device. For example, when the electronics device is in a stand-by state, its current consumption is as small as a leak current of an internal circuit (about 1 mA), which is extremely small. Meanwhile, when the electronics device is operating with a maximum load, its current consumption can be approximately 10 A. Therefore, in such a case, a current measuring circuit must be able to measure the current from 1 mA to 10 A, meaning that a dynamic range (a measurable range) of 10000 times is required. Since 14 bits are necessary to express the current from 1 mA to 10 A in a digital value, a 14-bit A/D converter is required for the current measuring circuit.
The A/D converter converts an input voltage into a digital value based on a ratio between the input voltage and a reference voltage. Therefore, a digital value Do of the 14-bit A/D converter can be expressed by the following expression (1), using an input voltage Vi and a reference voltage Vr.Do=(Vi/Vr)×16384  (1)
Minimum resolution of the 14-bit A/D converter is expressed by Vr/16384. It is 300 μV when the reference voltage Vr is 5.0 V, and 183 μV when the reference voltage Vr is 3.0 V. Hence, it is technically difficult to form the 14-bit A/D converter. Meanwhile, it is relatively easy to form a 10-bit A/D converter since minimum resolution of the 10-bit A/D converter is 29.3 mV when the reference voltage Vr is 3.0 V. However, a dynamic range (a convertible range) of the 10-bit A/D converter is only about 1000 times.
Supposing that the 14-bit A/D converter can be technically formed, a resistor value of a resistor element required to generate a voltage difference of 300 μV between both ends of the resistor element is 300 mΩ, when a current of 1 mA is flowing through the current path. When a current of 10 A is flowing through the current path, the voltage difference which is generated between both ends of the resistor element of 300 mΩ is 3.0 V, and power loss at this time is as much as 30 W. Therefore, the resistor value of the resistor element which is inserted in the current path in series needs to be small enough. In consideration of allowable power loss, the resistor value of the resistor element which can be used realistically has the limits of 3 to 5 mΩ. The voltage difference generated between both ends of the resistor element of 3 to 5 mΩ is 3 to 5 μV when a current of 1 mA is flowing through the current path, and is 30 to 50 mV when a current of 10 A is flowing through the current path. Since it is not realistic to make such a minute voltage to be the input voltage of the A/D converter, an amplifier for amplifying the voltage difference generated between both ends of the resistor element and outputting it to the A/D converter is provided at a pre-stage of the A/D converter.
In general, in order to form the A/D converter with relative ease, it is preferable to form it with its minimum resolution being about 3 mV. When a current of 1 mA is flowing through the current path, the voltage difference generated between both ends of a resistor element of 3 mΩ is 3 μV, and hence an amplifying factor of the amplifier needs to be approximately 1000 times. However, when a current of 10 A is flowing through the current path, the voltage difference generated between both ends of the resistor element of 3 mΩ is 30 mV, and hence an output voltage of the amplifier is as much as 300 V. Thus, it is necessary to increase an output bit number of the A/D converter (a bit number of the digital value obtained by A/D conversion) in order to increase the dynamic range of the A/D converter, but, when the bit number is increased, it is quite difficult to realize both of accuracy in the minimum resolution and increase of the dynamic range of the A/D converter.
According to Japanese Patent Application Laid-open No. Hei 6-90914, a dynamic range may be considered to be increased from the perspective that the overflow of the A/D converting circuit due to the DC noise is prevented, but a bit number of a digital value obtained by A/D conversion is not increased, and therefore the increase of the dynamic range is not realized consequently.